When measuring the nonlinearity distortion imparted by a device under test, the device under test is typically driven by a reference signal and the device under test provides an intermediate signal as a result. Upon transitions of the reference signal, differential changes in the intermediate signal are measured and compared to their expected values according to the intended transfer function of the device under test. As these differential changes are typically very small, the required measurement accuracy may be compromised when the signal to be measured is buried within the noise floor. Therefore, a notch and gain approach may be used to measure them. A difference and gain approach typically involves providing the intermediate signal to one terminal of a differential amplifier, and providing a base signal to the other terminal of the differential amplifier. As the base signal is close in amplitude to that of the intermediate signal, the base signal nearly cancels out (i.e. notches) the intermediate signal and the differential amplifier then amplifies the difference between the two signals and provides this amplified difference in the form of an output signal. Upon a transition in the reference signal, the resulting differential change in the output signal is measured rather than the intermediate signal, and compared to an anticipated value.
There are two traditional methods of providing the base signal. A first method involves connecting a variable bucking or Pedestal source as a base signal reference. This method is often used when testing the nonlinearity of a Digital-to-Analog Converter (DAC). To save testing time and expense for this type of DAC, it is common to test transitions only at the “major carries.” As is known in the art, “major carries” are transitions wherein the next major bit (on a digital reference signal) changes from a zero to a one. This occurs when the lower order bits are all ones and in the transition become all zeroes such that they have ‘carried over’ to the next adjacent higher order bit. For example, a transition from B0111 to B1000 is a “major carry” transition (i.e., for a four bit DAC with the Segmentation 1/3). When utilizing the “major carry” method, a variable source provides the base signal and is set to a value at or near the DAC's output value while the reference signal is one of the major carry values. The reference signal is then incremented (or decremented) and the differential change is measured through the notch and gain technique as described above. The problem with this method is that it requires an expensive second device (i.e., the variable source) capable of precisely generating a range of values at or near the major carry output value of the DAC, as well as maintaining this signal level/stability, over the entire measurement acquisition time.
A second method of providing a base signal gained popularity with the acceptance of segmented DACs. This second method involves using a second DAC, superior to the device under test, as the base signal reference source. In this method, the second DAC (i.e. the reference DAC) is driven by the same reference signal that drives the device under test. Because the reference DAC changes its output at the same time as the device under test, it is always supplying a reference signal at or near the output value of the device under test. In other words, the reference signal is stepping (i.e. bucking) in sync with the device under test. Accordingly, the reference DAC is often referred to as a “bucking DAC”. Unfortunately with respect to this method, the differential change measured by the notch and gain technique is now a combination of the differential changes of the outputs of both the device under test and the bucking DAC, because they each change at the same time in response to the reference signal. Therefore, the characteristics of the bucking DAC must be well known in order to isolate the nonlinearity of the device under test. Accordingly, the bucking DAC must have a higher resolution, be more stabile, have greater accuracy, and make less noise than the device under test.